Reticles
(eyepiece micrometers):
Everything you need to know

Reticles (pronounced
ret-eh-kuls, with the accent on the "ret") are sometimes also called
"eyepiece micrometers". They are clear circular glass inserts with a scale inscribed on
them (see first picture below). The reticle or eyepiece micrometer sits right at the focal plane inside the eyepiece
lens of the microscope and allows the investigator to make accurate
measurements of specimens.

If you have a stereo or binocular
microscope, there will only be a reticle in one of the
lenses. The lens that holds the reticle must have a reticle
retainer ring and the reticle must be of the proper diameter for the
particular eyepiece lens.

Straight line reticle

Reticle retainer ring removed

Reticle retainer ring in lens

Mounting the reticle is a
somewhat critical operation. They are relatively simple to
install but since they sit right on the
focal plane, any dust on the reticle will be quite apparent when
looking into the microscope. You can use a small spanner
wrench or simply a very small flat edge screwdriver. Insert
the blade into the notch in the ring and spin it out. Note:
Eyepiece lenses can be very different. On some lenses, the
ring is at the outer edge. On others, it may be deep inside
or you may have to unscrew a lens housing to get to the
ring. Be certain that you are removing the reticle retainer
ring and not a ring that secures the lens elements in place!
If in doubt, contact your microscope supplier. Once
the ring is removed, drop in the reticle then
re-install the retainer ring. We recommend using compressed
air to blow out any dust after installing your reticle.
Also, extra care
should be taken with the eyepiece lens whenever it is removed from
the microscope.

Reticles come in many
varieties and in different diameters. Most common is a straight line reticle (shown above).
This particular one is 10mm long with 100 divisions. It also has a cross
line at 5mm. Other types available include plain cross line, square grids
and circular patterns. Your choice of an eyepiece micrometer depends entirely on your
needs. If your microscope supplier doesn't have the exact one you
need, check with Microscope World, as they carry the most extensive supply of reticles online.They offer a wide selection of custom
reticles. You will have to supply the reticle diameter to be assured
of one that will fit.

The "actual"
distance between any two marks on the reticle are a function of
the objective lenses only. The best way to calibrate your
reticle is to use what is called a stage micrometer. This is a slide that has
tiny marks of a known dimensions inscribed on it. By making a
comparison of the marks on the stage micrometer to the marks on
the reticle, one can establish the actual value for each mark on
the reticle. Stage micrometers can be expensive and are also available from
microscope suppliers. A cruder less expensive method (for lower powers) is to use
a clear plastic metric ruler.

When you
look into your eyepiece lens, the markings will always be the same
but the size of the image superimposed under them will get larger
with more magnification. So, as you change to a
higher power objective lens, the represented value between marks will
change proportionately. For example, if each mark represents
0.1mm with a 1X objective lens, then with a 4X objective lens,
each mark will roughly represent 1/4 of 0.1mm or .025mm., which incidentally
is 25 micrometers (there are 1000 um in one milimeter).

For the most
accurate results, you should calibrate your reticle using a stage
micrometer for each objective lens. Even similar objectives
from the same manufacturer can vary by a few percent. As an
example, for a
reticle that is 10mm long with 100 divisions (as shown above),
each division was found to represent the
following distances:

1X objective (true
reading)

100um (0.1mm)

4X
objective

25um

10X
objective

10um

40X
objective

2.5um

100X
objective

1um

The proper way to
calibrate a reticle with a stage micrometer is to line up the left edges of
each, then see where the lines converge again. Lets say for an example, 22
lines from left to right on the reticle line up with 55 micrometers (actual
measurement) on the stage micrometer. So, 22 lines is to 55um as one line
is to X micrometers. This is a simple ratio problem and "X" can
be solved by dividing 55 by 22. Each line on the reticle then equals
2.5um. Once you calibrate your microscope for each power, record the
results so you won't have to do it again (unless your objective lens has a
correction collar). If you are calibrating a stereo zoom microscope, you
will have to recalibrate each time you change powers unless you are set at
positive click stops.

Finally, many people
purchase a separate eyepiece lens for their reticle so when they want to use
their microscope without it, they simply interchange the lenses.